    <meta charset="utf-8" />
    <meta content='text/html; charset=utf-8' http-equiv='Content-Type'>
    <meta http-equiv='X-UA-Compatible' content='IE=edge'>
    <meta name='viewport' content='width=device-width, initial-scale=1.0, maximum-scale=1.0'>

    {% if page.excerpt %}
    <meta name="description" content="{{ page.excerpt| strip_html }}" />
    <meta property="og:description" content="{{ page.excerpt| strip_html }}" />
    {% else %}
    <meta name="description" content="{{ site.description }}">
    <meta property="og:description" content="{{ site.description }}" />
    {% endif %}
    <meta name="author" content="{{ site.name }}" />

    {% if page.title %}
    <meta property="og:title" content="{{ page.title }}" />
    <meta property="twitter:title" content="{{ page.title }}" />
    {% endif %}

    <!-- for mathjax support -->
    {% if page.usemathjax %}
      <script type="text/x-mathjax-config">
        MathJax.Hub.Config({
        TeX: { equationNumbers: { autoNumber: "AMS" } }
        });
      </script>
      <script type="text/javascript" async src="http://cdn.mathjax.org/mathjax/latest/MathJax.js?config=TeX-AMS-MML_HTMLorMML"></script>
    {% endif %}